Rotate the entire image frame by selecting it, then dragging the rotate handle under the image from side to side. Hold the Alt-key to rotate in small snapped increments.
Note that Shape and Text frames will display this feature as well.
Frames may also be rotated from the General image attributes editor.
To rotate an image inside of its frame, use the Image angle slider under the General image attributes options.
Note that rotating an image within the frame can easily lead to empty corners around the image frame itself. This can be used for a fun effect - creating irregular bordered images.